Boudinage Classification

Journal of Structural Geology 26 (2004) 739–763 www.elsevier.com/locate/jsg Boudinage classification: end-member boudin types and modified boudin structuresq Ben D. Goscombea,*, Cees W. Passchierb, Martin Handa aContinental Evolution Research Group, Department of Geology and Geophysics, Adelaide University, Adelaide, S.A. 5005, Australia bInstitut fuer Geowissenschaften, Johannes Gutenberg Universitaet, Becherweg 21, Mainz, Germany Received 28 March 2002; received in revised form 25 August 2003; accepted 30 August 2003 Abstract In monoclinic shear zones, there are only three ways a layer can be boudinaged, leading to three kinematic classes of boudinage. These are (1) symmetrically without slip on the inter-boudin surface (no-slip boudinage), and two classes with asymmetrical slip on the inter-boudin surface: slip being either (2) synthetic (S-slip boudinage) or (3) antithetic (A-slip boudinage) with respect to bulk shear sense. In S-slip boudinage, the boudins rotate antithetically, and in antithetic slip boudinage they rotate synthetically with respect to shear sense. We have investigated the geometry of 2100 natural boudins from a wide variety of geological contexts worldwide. Five end-member boudin block geometries that are easily distinguished in the field encompass the entire range of natural boudins. These five end-member boudin block geometries are characterized and named drawn, torn, domino, gash and shearband boudins. Groups of these are shown to operate almost exclusively by only one kinematic class; drawn and torn boudins extend by no-slip, domino and gash boudins form by A-slip and shearband boudins develop by S-slip boudinage. In addition to boudin block geometry, full classification must also consider boudin train obliquity with respect to the fabric attractor and material layeredness of the boudinaged rock mass. Modified or complex boudin structures fall into two categories: sequential boudins experienced a sequence of different boudin block geometry components during progressive boudinage (i.e. continued stretch), whereas reworked boudins were modified by subsequent deformational episodes (folded, sheared and shortened types). Correct classification of boudins and recognition of their modification are the crucial first stages of interpretation of natural boudin structures, necessary to employing them as indicators of shear sense, flow regime and/or extension axes in terranes otherwise devoid of stretching lineations. q 2003 Elsevier Ltd. All rights reserved. Keywords: Boudin; Boudinage; Shear zones; Modified boudin; Kinematic indicator 1. Introduction Simpson and Schmid, 1983; Hanmer, 1984, 1986; Van der Molen, 1985; Ramsay and Huber, 1987; Goldstein, 1988; Boudinage is the disruption of layers, bodies or foliation Lacassin, 1988; DePaor et al., 1991; Hanmer and Passchier, planes within a rock mass in response to bulk extension 1991; Swanson, 1992, 1999). Nevertheless, there has been along the enveloping surface. Boudin structures were first no attempt to erect a classification scheme for the full range described by Ramsay (1881) and named by Lohest (1909). of natural boudin structures. We propose a simple The huge variety that occur in nature have been the subject classification scheme that accounts for all simple unmodi- of considerable study (Wilson, 1961; Ramsay, 1967; fied boudin geometries that have been described and further, Etchecopar, 1974, 1977; Hobbs et al., 1976; Lloyd and relate these end-member boudin geometries in a hierarchical Ferguson, 1981; Lloyd et al., 1982; Blumenfeld, 1983; scheme that considers their relationship to the three kinematic classes by which boudinage occurs (Table 1; q Extended datasets, Appendices B–C, can be found in the online version Goscombe and Passchier, 2003). These kinematic classes of this paper are: (1) symmetrical boudins, which do not experience slip * Corresponding author. Correspondence address: Geological Survey of Western Australia, P.O. Box 1664, Kalgoorlie, W.A. 6433, Australia. on the inter-boudin surface, forming by no-slip boudinage, E-mail address: [email protected] (B.D. Goscombe). and two asymmetrical classes with slip on the inter-boudin 0191-8141/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2003.08.015 740 B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 Table 1 Classification of boudinage of monoclinic symmetry Below the thick shaded line are real boudin structures observable in rocks and above it, their correlating kinematic class of formation. Only boudin types that have not been modified are included here. All natural boudin structures can be classified in terms of the five end-member boudin block geometries that are highlighted by shading. These can be further sub-divided and the common examples are given (see also Fig. 3). surface: (2) synthetic slip boudinage (S-slip) and (3) 2. The boudin dataset antithetic slip boudinage (A-slip) with respect to bulk shear sense. Our dataset contains over 2100 boudins and collates the In this paper, we classify natural boudin structures, both most important geometric parameters and features describ- simple and modified, according to boudin block geometry, ing boudin structures illustrated in Fig. 1 and defined in boudin train geometry and nature of the boudinaged Appendix A. A summary of the dataset is available in material. This analysis is based on an extensive dataset, Table 2. Data were obtained primarily from the authors’ collected by the authors and collated from the literature, of field studies in a wide range of geological environments; quantitative data and qualitative features that describe including non-coaxial shear environments (Zambezi Belt in boudin structures from a wide range of geological Zimbabwe; Ugab Terrane, Damara Orogen and Kaoko Belt environments, spanning different flow regimes, rheological in Namibia; and the Himalayas in Nepal), pure shear contrasts, metamorphic grade and strain. The boudin environments (Adelaidean Fold Belt in South Australia) and structures involved have been analysed in two integrated transtensional environments (oceanic crust on Macquarie ways. First, natural groupings have been delineated based Island). These data are augmented by measurement of on boudin block geometries, boudin train geometries and photographs of boudins in true profile (normal to the boudin the nature of the boudinaged material that can be recognized long axis) from the authors’ collections and from the in the field. Second, we have related geometric groups to literature (Wilson, 1961; Ramsay, 1967; Weiss, 1972; bulk flow and kinematic groups wherever possible. We Hobbs et al., 1976; Lloyd and Ferguson, 1981; Borradaile largely restrict our study to boudin structures of monoclinic et al., 1982; Lloyd et al., 1982; Ramsay and Huber, 1983, symmetry, considering the 2D-geometry in the profile plane 1987; Sengupta, 1983; Simpson and Schmid, 1983; Bos- normal to the long axis of the boudin. Where necessary, 3D worth, 1984; Fry, 1984; Hanmer, 1984, 1986; Mullenax and effects are included. We use the results of this analysis to Gray, 1984; Van der Molen, 1985; Davis, 1987; Goldstein, establish criteria that define simple end-member boudin 1988; Malavielle and Lacassin, 1988; Hanmer, 1989; types and subsequently recognize these components within Laubach et al., 1989; Passchier et al., 1990; DePaor et al., complex modified boudin structures. The latter can be 1991; Goscombe, 1991, 1992; Hanmer and Passchier, 1991; either: (1) reworked boudin structures modified by sub- Swanson, 1992; Carreras and Druguet, 1994; Yamamoto, sequent deformation unrelated to boudinage or (2) sequen- 1994; Daniel et al., 1996; Lisle, 1996; Conti and Funedda, tial boudin structures that contain multiple end-member 1998; Kraus and Williams, 1998; Roig et al., 1998). Data boudin type components and formed by progressive from photographs comprise 60% of the total dataset. The boudinage. photographs do not permit the measurement of parameters B.D. Goscombe et al. / Journal of Structural Geology 26 (2004) 739–763 741 3. Boudin structural elements 3.1. General parameters Boudins are complex and highly variable 3D structures and so to be uniquely described requires a large number of quantifiable dimensional and angular parameters relating the structural elements (Fig. 1; Appendix A). No consistent nomenclature for boudin structural elements or geometric parameters has been adopted in the literature, even for the simplest system of symmetric boudinage (Jones, 1959; Wilson, 1961; Hobbs et al., 1976; Penge, 1976; Lloyd et al., 1982). Thus, a suite of nomenclature for structural elements and geometric parameters that uniquely describes all possible boudin structures is adopted in this study (Fig. 1) and defined in detail in Appendix A. A description of the nature of boudinage involves first of all the overall nature of the boudinaged element. The nature or degree of layeredness of the boudinaged material forms a continuous spectrum that can be classified into the following. (1) Object boudinage of a competent object of limited dimensional extent such as a mineral grain. (2) Single-layer boudinage of a competent layer in a less competent host (Ghosh and Sengupta, 1999). (3) Multiple- layer boudinage of a packet of thin competent layers (Ghosh and Sengupta, 1999). A variation is ‘composite boudins’ composed of boudinaged sub-layers within a boudinaged Fig. 1. Nomenclature and symbols of boudin structural elements and packet of layers, giving nested boudins of different scale geometric parameters.

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